The long-term goal of my research is to understand how bacteria influence the behavior of the eukaryotes with which they co-exist. Animal-bacterial associations are ancient and ubiquitous, ranging from host-pathogen to symbiotic relationships. How have these long-standing relationships shaped the evolution of each partner? What is the nature of microbial signals that elicit morphogenic responses in eukaryotes and how do they function? I propose to study a special example of a eukaryotic-bacterial interaction in which Algoriphagus bacteria induce colony formation in the choanoflagellate Proterospongia sp. The approach proposed here is the first attempt to identify and characterize signals involved in choanoflagellate morphogenesis. First, I will isolate the soluble factor required for colony induction using traditional biochemical techniques. The nature of this molecule will be analyzed by mass spectrometry. Second, a genetic approach will be taken to identify PR1 genes involved in choanoflagellate colony induction. I will generate and screen PR1 transposon libraries for mutants that fail to elicit Proterospongia colonies. I will also screen PR1 expression libraries for colony inducing activity in a bacterial species that does not normally elicit colony formation. Third, I will characterize the function of candidate PR1 colony induction bacterial genes and pathways by determining the expression patterns and regulation of candidate gene products. This research has the potential to uncover molecules and genes important for colony development in the closest known relatives of animals and has implications for understanding modern host-bacterial interactions, the evolution of pathogenesis and immunity, and the origin of animal multicellularity. PUBLIC HEALTH RELEVANCE: We are only beginning to appreciate the myriad of ways the bacteria that live within us contribute to our development and physiology. Bacteria are beneficial to maintaining a healthy immune system and regulating our metabolism, but gaining a precise understanding of the role they play in our bodies has proven difficult. By studying the relationship between choanoflagellates, the closest known living relative of animals, and their bacterial food source, we can begin to understand how bacteria came to influence so many of our fundamental processes.